This invention relates generally to derrick hooks used on rotary earth drilling rigs and, more particularly, to an improved shock absorbing snubber of the type used to dampen the vertical movement of the hook shank.
Rotary earth drilling rigs used for drilling oil and gas wells utilize a traveling block and hook arrangement for tripping stands of pipe into and out of the well bore. When raising the pipe stand, an elevator assembly is positioned around the box end of the pipe held by slips in the rotary table. A pair of steel links connect the elevator assembly to the derrick hook. The derrick hook and associated traveling block are hoisted along the vertical axis of the drilling rig by cable linkages.
Pipe strings at times extend thousands of feet into the earth requiring derrick hooks of massive size. Prior art hooks typically have a shank which is free to move up and down within an outer housing. A helical coil spring is positioned around the shank in the hook housing. A spring seat is connected to the upper end of the shank and engages the coil spring. Downward vertical movement of the shank, such as occurs when a load is applied to the hook, compresses the spring.
In raising a stand of pipe from the well bore, the slips are released and the weight of the pipe string causes the hook shank to move downward along the vertical axis of the housing. This action compresses the coil spring positioned about the shank. When the pipe stand has been raised to the proper position, the slips are again engaged at the rotary and the pipe is unscrewed. As the threaded tool joint connection is unscrewed, the coil spring in the derrick hook exerts an upward force on the pipe stand. This upward force assists in unscrewing the connection and in raising the stand of pipe high enough to allow it to be moved sidewise into the racking area.
In order to prevent the pipe from bouncing or "jumping out" as the spring tension is relieved, a shock absorbing unit or "snubber" is utilized to dampen the vertical movement of the hook shank. By dampening the vertical travel of the shank, a controlled rate of return is achieved which prevents damage to the threaded tool joint connections.
In certain prior art embodiments, the snubber unit is physically connected to the hook shank. One such snubber assembly uses a stationary plunger and a barrel that slides telescopingly over the plunger. The barrel is secured to the top of the shank. The plunger and barrel contain hydraulic fluid. An orifice in the bottom of the plunger restricts fluid flow between the barrel and plunger, dampening both upward and downward movement of the shank. This design is difficult to service since it is necessary to disassemble the entire hook in order to service the snubber. Also, application of a sudden load to the hook shank is transferred directly to the snubber assembly since the barrel is secured to the top of the shank.
Other prior art snubbers are self-contained units not physically connected to the hook shank. One such design includes a piston located in a chamber in which hydraulic fluid exerts continuous hydrostatic pressure against the cylinder walls. The piston is biased downwardly and has a check valve that allows free fluid flow past the piston when moving downwardly. When moved upwardly, only restricted flow past the piston is allowed. Upward movement of the shank pushes the piston upwardly, dampening shank movement only in the upward direction, as desired. The continuous pressure, however, in the chamber results in some leakage problems around the piston shaft.